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Abstract Plant organ size is an important agronomic trait tightly related to crop yield. However, the molecular mechanisms underlying organ size regulation remain largely unexplored in legumes. We previously characterized a key regulator F‐box protein MINI ORGAN1 (MIO1)/SMALL LEAF AND BUSHY1 (SLB1), which controls plant organ size in the model legumeMedicago truncatula. In order to further dissect the molecular mechanism, MIO1 was used as the bait to screen its interacting proteins from a yeast library. Subsequently, a KIX protein, designated MtKIX8, was identified from the candidate list. The interaction between MIO1 and MtKIX8 was confirmed further by Y2H, BiFC, split‐luciferase complementation and pull‐down assays. Phylogenetic analyses indicated that MtKIX8 is highly homologous toArabidopsisKIX8, which negatively regulates organ size. Moreover, loss‐of‐function ofMtKIX8led to enlarged leaves and seeds, while ectopic expression ofMtKIX8inArabidopsisresulted in decreased cotyledon area and seed weight. Quantitative reverse‐transcription PCR and in situ hybridization showed thatMtKIX8is expressed in most developing organs. We also found that MtKIX8 serves as a crucial molecular adaptor, facilitating interactions with BIG SEEDS1 (BS1) and MtTOPLESS (MtTPL) proteins inM. truncatula. Overall, our results suggest that the MIO1‐MtKIX8 module plays a significant and conserved role in the regulation of plant organ size. This module could be a good target for molecular breeding in legume crops and forages.